Acoustical Physics最新文献

A consistent theory of sound generation in a turbulent boundary layer developing over a flat smooth boundary at low Mach numbers is presented. The main source of sound and the long-wavelength part of pressure fluctuations on the boundary are incoming shear (viscous) waves generated by Lighthill quadrupoles in the near-wall region of the turbulent boundary layer. It is shown that with an increase in the Reynolds number (decrease in viscosity), the role of viscosity in sound generation does not decrease, but instead increases. Quantitative estimates of the spectrum of the sound power density generated in a turbulent boundary layer are given.

Two-dimensional linearized hydrodynamic equations describing wave propagation in a stratified heavy gas are considered. The hydrodynamic equation system is reformulated as a single Schrödinger type operator equation. Waves with (beta = frac{{{{l}_{z}}}}{{{{l}_{x}}}} ll 1) are considered, where ({{l}_{z}}) and ({{l}_{x}}) are the characteristic vertical and horizontal scales, respectively, and study the asymptotic behavior of solutions as (beta to 0). It is shown that the set of solutions depending on (beta ) form two disjoint classes. For solutions from each of the selected classes, its own, asymptotic as (beta to 0) , approximate equation system is proposed. The selected classes of solutions are acoustic and internal gravity waves. It is shown that the hydrodynamic variables of acoustic and gravity waves are related by certain stationary relationships, different for each class. This makes it possible to formulate the problem of separating the contributions of acoustic and gravity waves in the initial condition. The existence of a solution to this wave separation problem is shown. Examples of solving the problem of dividing the general problem into subproblems on the propagation of acoustic and gravity waves are given. Estimates for the division of the energy of the initial perturbation by wave type are obtained.

In this study, correlation reception of thermal acoustic radiation by a pair of sensors was carried out. The experiment used receivers with different bandwidths, varied the size of the heated sources and the distance from the sources to the receivers, and also shifted the sources in the transverse direction perpendicular to the acoustic axis of the system. For each case, applying the relations used in radio astronomy, the correlation functions of thermal acoustic radiation were calculated. It is shown that the experimentally obtained and calculated cross-correlation functions are close, taking into account the measurement error.

The article presents the results of a laboratory experiment testing a method for reconstructing a sound field excited by a calibrated source in the free space from measurements of a field excited by the same source in a tank with reflecting boundaries. The reconstruction procedure uses a standard acoustic monopole and compares the fields emitted by it from specially selected points of the tank with the field of the calibrated source. In the experiment, the frequency dependence of the field intensity of the calibrated source averaged over a sphere of large radius was evaluated.

One of the episodes of the ASIAEX 2001 experiment (in the South China Sea) is considered, in which a large internal wave soliton moved along two stationary acoustic paths 32 and 19 km long, and associated fluctuations in the intensity of low-frequency sound (224 and 300 Hz) were observed. During the study, the phenomenon of constancy of the dominant frequency of fluctuations over time was discovered. For example, during 6-h soliton motion along a long path, where the sea depth changed three times (from 350 to 120 m), and the soliton velocity, two times (from 2 to 1 m/s), the dominant frequency of fluctuations remained approximately constant at 1.5 cph (cycles per hour) with an accuracy of 10%. The paper analyzes the causes of this phenomenon. For this, the soliton is considered within the framework of a two-layer model of the aquatic environment, and sound propagation, within the framework of mode and ray theories. According to ray theory, the dominant frequency of fluctuations is determined by the ratio of the soliton velocity to the ray cycle responsible for the dominant fluctuations. In mode theory, a similar expression is obtained where the role of the ray cycle is played by a combination of spatial beat periods of several pairs of modes. It is shown that with a change in the sea depth, the soliton velocity and the ray cycle change almost proportionally, as a result of which the dominant frequency of fluctuations remains constant. The described phenomenon may be universal and not limited to the ASIAEX water area. The constancy of the dominant frequency allows one, in particular, to determine the variable soliton velocity as a function of time or distance, which is successfully demonstrated in the work and can be used for acoustic monitoring of solitons.

The problem of acoustic wave propagation with thermoviscous boundary conditions is studied. For thermoviscous boundary conditions, a time-domain formulation is formulated based on the concept of a fractional derivative. A weak formulation of the problem is given, which is reduced to a system of Volterra-type integro-differential equations using the finite element method. An implicit finite-difference scheme is constructed for the numerical solution of this system. To verify it, the problem of sound propagation in a thin pipe is modeled, and the results of numerical modeling are compared with the analytical solution